The present invention relates to an image pickup element, a plurality of image pickup elements constituting a solid-state image sensing device of a CCD (i.e. Charge Coupled Device) type, and relates to a method for manufacturing the image pickup element.
There have been conventionally proposed various solid-state image sensing devices of a CCD (i.e. Charge Coupled Device) type. The CCD type solid-state image sensing device is composed of a large number of image pickup elements which are formed on a single piece of substrate, for example, in a linear or matrix form.
FIG. 4 is a figure showing a structure of a conventional image pickup element 200. The light receiving part 13 transforms an incident light into an amount of electric charge which is compliance with the intensity of the incident light. The P-type well layer 12 discharges an excess amount of electric charge generated in the light receiving part 13 when an excess amount of light is incident on the light receiving part 13, to the N-type semiconductor substrate 11. Namely, the P-type well layer 12 functions as a vertical type overflow drain. The electric charge readout part (which is also referred to as transfer gate part) 23 reads out the electric charge generated in the light receiving part 13. The CCD layer (which is also referred to as electric charge transfer part or as CCD register) 15 outputs the electric charge, of the light receiving part 13, which is read out by the electric charge readout part 23 to an output circuit (unshown). The barrier layer 14 under the CCD layer 15 prevents an exchange of electric charge between the N-type semiconductor substrate 11 and the CCD layer 15. The surface P+ layer 16 includes a P+ impurity layer 16a for separating between the adjacent image pickup elements, and the surface P+ layer 16 controls the dark current of the light receiving part 13. Depending upon the amount of the P-type impurity doped in the impurity layer 17, the threshold value of the electric charge readout part 23 is controlled.
A description is made below upon how to manufacture the image pickup element 200, in brief.
Firstly, after the P-type well layer 12 is formed on the N-type semiconductor substrate 11, the CCD layer 15 made of the N-type impurity, the barrier layer 14 made of P-type impurity under the CCD layer 15, and the P+ impurity layer 16a for separating between the adjacent image pickup elements, are formed inside the P-type well layer 12. And then the P-type impurity layer 17 for controlling the threshold value of the electric charge readout part 23 is formed by a boron radiation. The doping of the P-type impurity by this boron radiation, is performed with respect to the whole region in which the image pickup element is formed on the substrate 11.
After the above process, a silicon oxide film 18, a silicon nitride film 19, a gate electrode 20, and a silicon oxide layer 21 covering the gate electrode 20, are formed thereon. Also, a second gate electrode (unshown) is formed thereon. Then, the light receiving part 13 is formed by doping the N-type impurity to a rectangular region which is defined by the gate electrode 20 and the second gate electrode. In the same way, the surface P+ layer 16 is formed by making use of the self-alignment of the gate electrode. Incidentally, the surface P+ layer 16 is connected to the P+ impurity layer 16a which has already been formed. The surface P+ layer 16 is grounded outside a region in which a large number of the image pickup elements are arranged, and the electrical potential of the surface of the light receiving part 13 is made equal to a GND electrical potential. Then, the image pickup element 200 is accomplished by forming the light blocking metal layer 22 with a dry etching.
When an image is read by the solid-state image sensing device (so-called a CCD tip) which is composed of a large number of the image pickup elements 200 formed on a single piece of substrate, and when the image thus having been read out is shown on a display device, a blank pixel appears on a reproduction image of the display device in correspondence to a location at which the image pickup element with malfunction exists. The blank pixel is not desirable. In order to form the light receiving part 13 of the aforementioned conventional image pickup element 200, the N-type impurity is doped therein. Hereat, the amount of the N-type impurity, performing an actual photoelectric conversion, is decreased due to the effect of the P-type impurity layer 17 having already been doped, and this has been one factor causing the malfunction.
In order to solve the aforementioned technical problem, there has been proposed a formation of the impurity layer 17 for controlling the threshold value, in correspondence to a location only at which the impurity layer 17 functions as the electric charge readout part 23 under the gate electrode 20, during manufacturing of the image pickup element 200 (refer to Japanese Unexamined Laid-Open Patent Publication No. 1-281764).
However, when the impurity layer 17 is formed only at the location at which the layer 17 functions as the electric charge readout part 23 under the gate electrode 20, it is necessary to prepare a resist mask at the location where the impurity is doped, and therefore it is necessary to set the gate electrode larger (or longer) to some extent. Under the situation that the miniaturization of the image pickup element is desired, making the gate electrode larger (or longer) by taking any deviation etc., relating to accuracy in manufacturing, of the mask into consideration, is not desirable because of incurring the decrease of the capacity of the light receiving part 13.